The present invention relates to a microwave oscillator using a FET. Microwave circuits including a high frequency transistor such as a GaAs FET or the like are usually formed with a planar circuit structure. This invention provides a new and improved construction of such a microwave oscillator formed with a planar circuit substrate.
FIGS. 1 through 5 show examples of structures of conventional oscillator circuits. With reference first to FIG. 1, the planar circuit structure of the conventional structure is formed by metal films, shown by shadowed sections in the drawings, disposed on a first surface of a substrate 1 which is made of alumina porcelain or the like. In addition, a ground conductor which is made of metal film is disposed over the entire second surface of the substrate 1 or at least on a part of the second surface opposite to the metal film disposed on the first surface of the substrate 1. When alumina porcelain is used as the substrate 1, a substrate of about 0.6 .mu.mm thick is generally used. Metal films are usually formed by providing a layer of chromium having thickness of 500-1000 A on the alumina porcelain, then coating it with layer of gold 4-5 .mu.m thick with a vacuum evaporating or electroplating technique. A photo-etching technique, eliminating unnecessary metal films, is generally used for forming the planar circuit structure.
Referring now to FIG. 1, a GaAs FET 2 has three electrodes, namely a drain, gate and source. A transmission line 3 is connected to the gate electrode of the GaAs FET and a negative bias voltage is applied thereto. A transmission line 4 is connected to the source electrode of the GaAs FET and supplied with a zero bias voltage while a transmission line 5 is connected to the drain electrode of the GaAs FET and supplied with a positive bias voltage. The bias voltage supplying circuit is not shown. The oscillating frequency of such an oscillator is primarily determined by the length l.sub.1 of the gate transmission line 3 which is selected to be one-half of the wavelength corresponding to the oscillating frequency. The length l.sub.2 of the source transmission line 4 is selected to be a quarter-wavelength. The oscillating output is taken out from the drain transmission line 5.
The three electrodes the of GaAs FET may otherwise be connected to the transmission line other than with the connections described above. That is, the gate electrode of the GaAs FET may be connected to the line 3, the drain electrode of the GaAs FET to the line 4 and the source electrode of the GaAs FET to the line 5. Furthermore, for powering the oscillator, it is possible to use a single bias voltage source instead of two bias sources, one positive and the other negative as mentioned above. To this effect, the gate electrode of the GaAs FET is grounded and a suitable resistance, for example 10.OMEGA., is disposed between the gate and source electrode of the GaAs FET and a positive voltage may be applied to the drain electrode of the GaAs FET. In the oscillator thus described, the external Q of the circuit is low (about several tens), much noise is generated, and the oscillator is easily affected by fluctuations in the load, voltage, ambient temperature and the like. Because of large changes of frequency caused by such fluctuations, there are difficulties in using such as oscillator.
FIG. 2 through FIG. 4 illustrate techniques for increasing the external Q and frequency stability of the oscillator. Referring to FIG. 2, a dielectric resonator 6 is disposed near a output transmission line 5. This dielectric resonator 6 acts as a band rejection filter (BRF) which stabilizes the oscillating frequency of the oscillator when the oscillating frequency determined by the length l.sub.1 of gate transmission line 3 is close to the resonant frequency of the band rejection filter. In this way, a GaAs FET oscillator operating in a 11 GHz output band may have an external Q of more than 3000 and a frequency stability of .+-.300 KHz against environmental temperature variations from -10.degree. to +50.degree. C. A frequency change of about 50 to 60 MHz would be observed over the same temperature change without the BRF. In FIG. 3, showing a side view of FIG. 2, a metal plate 7 is spaced from a surface of dielectric resonator 6. The distance d between the dielectric resonator 6 and the metal plate 7 may be adjusted. (The metal plate 7 is not shown in FIG. 2 nor is a case containing a oscillator assembly and supporting the metal plate 7.) The resonant frequency of the dielectric resonator 6 may be adjusted by changing the distance d between the metal plate 7 and the resonator 6. In order to stabilize the oscillating frequency of the oscillator, it is necessary to use a dielectric resonator having a resonant frequency close to, and preferably somewhat lower than, the oscillating frequency determined by the length l.sub.1 of the gate transmission line, and the distance d should be adjusted to obtain a suitable external Q. In FIG. 3, numeral 8 refers to a ground conductor.
FIG. 4 shows another example in which a dielectric resonator is placed near the gate transmission line 3. With the arrangement of FIG. 4, high stability of the oscillation is obtained.
In a conventional oscillator as shown in FIG. 1 through FIG. 4, the oscillating frequency is fundamentally determined by the length l.sub.1 of the gate transmission line. Therefore, the conventional oscillator is not suitable for practical use since performing fine adjustment is very difficult after the planar circuit pattern has been fixed. Also, stabilization of the oscillator by means of a BRF is difficult in that the output of the oscillator decreases when both the external Q and the stability increase. Moreover, even through the output power of the oscillator is about 40 mW without a BRF, the output power drops below 10 mW if the external Q is increased by above about 2000 by using the BRF. In addition, it is difficult to produce a number of oscillators having the same performance because the location of the BRF, that is, the distance from the FET to the BRF and from the BRF to the line 5 strongly influences the characteristics of the oscillator, such as frequency, output power and the like. Yet further, a conventional oscillator has problems in production in that the characteristics of the oscillator are significantly affected by the combination of the line lengths l.sub.1 and l.sub.2.
FIG. 5 shows an example of a conventional oscillator intended for solving such problems. In FIG. 5, a source transmission line 4 is connected to a ground conductor through an aperture 9 which is formed in the substrate 1 near the FET 2. Oscillating output is taken out from a drain transmission line 5. A part of the output power which is taken out from the drain transmission line 5 through a line 51 is transmitted to a gate transmission line 3 by way of a dielectric resonator 6. In this case, the FET 2 acts as an amplifier amplifying repeatedly signals having a frequency able to pass the dielectric resonator 6, that is, the resonant frequency of the dielectric resonator. Therefore, as the stability of the frequency is determined by the stability of the dielectric resonator, it is theoretically possible to obtain an oscillator with high stability. Fine adjustment of the oscillating frequency may be also easily provided by adjusting the resonant frequency of the dielectric resonator as shown in FIG. 3.
Nonetheless, with the arrangement of FIG. 5, it is difficult to properly design the circuit. The oscillating operation strictly depends upon the length and structure of the gate transmission line 3 and the branch line 51 and the distances between the dielectric resonator 6 and these lines 3, 51. Yet further, oscillation at a frequency different from the predetermined frequency can sometimes occur. For these reasons, there are yet many difficulties in producing a oscillator which can be put into practical operation.